As one of optical signal transmission schemes, for example, there is a digital coherent light transmission scheme. The digital coherent light transmission scheme uses, as modulation schemes for polarization-multiplexed optical signals, multi-level modulation schemes such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (16-QAM). Improvement of the performance of digital signal processors (DSPs), analog-digital converters, and digital-to-analog converters in recent years has made it possible to select one scheme from a plurality of multi-level modulation schemes.
The selection of the multi-level modulation schemes is made in consideration of a transmission distance of a polarization-multiplexed optical signal and various penalties that are generated in a transmission path. Factors of this kinds of penalties include amplified spontaneous emission (ASE) noise, a nonlinear optical effect such as self-phase modulation, polarization mode dispersion (PMD), and polarization dependent loss (PDL).
The penalty is measured as, for example, the bit error rate (BER) of the polarization-multiplexed optical signal in a receiving device as a result. For example, a difference between an actually measured BER and the limit value (for example, 1.0×10−2) of error correction capability of a forward error correction (FEC) code is regarded as a margin in a transmission system. In the selection of the multi-level modulation schemes, for example, the BER in each multi-level modulation scheme is measured, and from the measurement results, a multi-level modulation scheme providing the smallest margin is selected so that the highest possible transmission speed is achieved.
For example, as the related art, Japanese Laid-open Patent Publication No. 2015-188165 and Japanese Laid-open Patent Publication No. 2004-138615 have been published.
The PMD is varied with temporal probability due to birefringence that is caused by external stress or distortion of a core of an optical fiber, for example. Thus, in the selection of the multi-level modulation schemes, the margin is desirably provided in consideration of the maximum value of the PMD.
In view of this, if BER measurement is performed for a long period of time (for example, a few months), the maximum value of the PMD may be measured, but it is difficult to take such a long period of time to set the transmission system in reality. The maximum value of the PMD is accordingly estimated based on a design value such as the specified value of the optical fiber to be used in the selection of the multi-level modulation schemes.
The accuracy of estimate of the PDM based on the design value is, however, not high, and an inappropriate multi-level modulation scheme may be selected, with the result that a margin excessively larger than the actual one may be provided.
In view of the foregoing, a transmission device and a selection method for a modulation scheme that may select an appropriate multi-level modulation scheme are desirably provided.